A semiconductor package has a resin-encapsulation portion to protect a semiconductor element. For formation of the resin-encapsulation portion (for encapsulation of a semiconductor element), a curable resin e.g. a thermosetting resin such as an epoxy resin is used.
As a method for producing a semiconductor package, for example, a method comprising an encapsulating step by a so-called compression molding method or transfer molding method is known, wherein a substrate having a semiconductor element mounted thereon, is disposed so that the semiconductor element is located at a predetermined position in the cavity of a mold, and a curable resin is filled in the cavity to form a resin-encapsulation portion. In such a method, in order to prevent adhesion of the resin-encapsulation portion and the mold, usually, a mold release film is disposed on the cavity surface of the mold.
As one of processes for producing semiconductor packages, there is a process which comprises a step (collectively encapsulating step) of mounting a plurality of semiconductor elements on a substrate, and collectively encapsulating such semiconductor elements with a curable resin, to obtain a collectively encapsulated product comprising the substrate, the plurality of semiconductor elements and a resin encapsulation portion, and a step (singulation step) of cutting and singulating the resin-encapsulation portion and the substrate of the collectively encapsulated product, so that the plurality of semiconductor elements are separated, to obtain a plurality of semiconductor packages (e.g. Patent Document 1). This process is excellent in productivity and therefore is widely used.
In the step of encapsulating the semiconductor elements, a mold release film may sometimes be disposed on the cavity surface of the mold in order to prevent adhesion of the curable resin and the mold. Especially in a case where a plurality of semiconductor elements are collectively encapsulated, as compared with the case of encapsulating one semiconductor element in one cavity, the cavity tends to be large-sized and complicated so that the resin-encapsulation portion tends to be hardly releasable, and therefore, a mold release film is used in many cases.
At the time of encapsulation, the mold release film is stretched along the cavity surface of the mold by vacuum suction and brought in such a state as closely in contact with the cavity surface. At that time, however, if the mold release film is brought in contact with the cavity surface in such a state that air is not yet completely evacuated during stretching, air spaces may partly be formed between the mold release film and the cavity surfaces, and wrinkles are likely to be formed in the mold release film at such portions. If the mold release film has wrinkles, the shape of such wrinkles on the mold release film is likely to be transferred to the surface of the resin-encapsulation portion, thus leading to poor appearance, whereby the yield will be lowered.
In order to overcome such a problem, it has been proposed to adjust the surface roughness Rz on at least one side of the mold release film to be at least 3.0 μm (Patent Document 2). It is disclosed that by mounting the mold release film on the mold so that the surface having a surface roughness Rz of at least 3.0 μm will be in contact with the mold side, it is possible to prevent formation of wrinkles.
Further, in the production of a semiconductor package, it is common to form an ink layer on the surface of the formed resin-encapsulation portion by printing with an ink, in order to display information such as the product number, manufacturers, etc.
If the adhesion between the resin-encapsulation portion and the ink layer is low, the ink layer tends to peel from the resin-encapsulation portion as the time passes.
In order to improve the adhesion between the resin-encapsulation portion and the ink layer, it has been proposed to use a mold release film having irregularities formed on the surface to be in contact with the resin thereby to increase the surface roughness, and to form a resin-encapsulation portion by disposing the mold release film on the mold so that the irregularities face the curable resin side (e.g. Patent Document 3). In this case, the irregularities on the surface of the mold release film in contact with the resin are transferred to the surface of the resin-encapsulation portion. By the presence of such irregularities, the adhesion of the ink layer to the resin-encapsulation portion is improved.